Double photoionization of magnesium from threshold to 54 eV photon energy

We have measured the relative single and double photoionization cross section of magnesium from 22 to 54 eV photon energy using monochromatized synchrotron radiation. The corresponding double-to-single ionization ratio as well as the relative single photoionization cross section compares reasonably well with recent theoretical calculations [A. S. Kheifets and I. Bray, Phys. Rev. A 75, 042703 (2007)]. The photon energy dependence of the ratio can be modeled by a suitably scaled helium double-to-single photoionization ratio. However, to our surprise, a previously proposed scaling model for that ratio does not work in the case of Mg. From the near-threshold double-photoionization cross section we estimate the range of validity of the Wannier threshold law to be about 0.7 eV with a rather small cross section near threshold.